In internal combustion engines, as a method of reducing harmful components in the exhaust gas, there is known a feedback control method wherein a concentration of oxygen in the exhaust gas is detected as an exhaust component value, or an amount of intake air and a rotating speed of the engine are also detected, and an air-fuel ratio of an air-fuel mixture produced by a carburetor or fuel injection apparatus is controlled to a value in a predetermined theoretical air-fuel ratio range on the basis of the detection signals. Using this method, the ternary catalyst which reduces the harmful exhaust components can be made to function effectively.
However, in the case of controlling the air-fuel ratio of the air-fuel mixture to a value in the theoretical air-fuel ratio range using such a feedback control method, if the temperature of the intake air increases due to an increase in the temperature of the outside air, the amount of intake air necessarily decreases and, due to the given air-fuel ratio, the absolute amount of fuel supplied to the engine is reduced. Thus, there is a drawback, in that the rotating speed of the engine is thereby decreased in the idle operating state, causing stability in idling to deteriorate.
To eliminate this drawback, hitherto there has been provided an actuator to open an intake throttle value arranged in the intake passage by a predetermined opening degree in the idle operating state in the case where the intake air temperature was high. Alternatively, there has been provided a bypass air passage to bypass the intake throttle valve and a solenoid valve to open the bypass air passage in the idle operating state in the case where the intake air temperature was high. Due to this, the intake air quantity is increased in the idle operating state when the intake air temperature is high, thereby avoiding the reduction in the absolute amount of fuel due to a decrease in the amount of intake air. In this way, a decrease in the rotating speed of the engine is prevented and idling stability is secured. However, as mentioned above, this method needs a control component such as the actuator, solenoid valve or the like to increase the amount of intake air, causing the structure to be complicated and resulting in an increase in cost.